I'm a salesperson and can relate to all sides. I think there are as many types of salespeople as there are consumers. The trick is to make the right match.
Being in sales, when I make a big ticket purchase, I've done my research and would just like to make my purchase. I don't want to be "sold" anything more or different than what I came in for. I work with a girl that can sell ice to eskimos and she would make me crazy if she were selling to me but boy does she sell alot! I'm amazed at how easily lead so many people are. No minds of their own!
Regarding the white couch, I probably would have said "Wow you are brave! That wouldn't be white a nanosecond in my house!" I might suggest how to care for it but that's about it. I wouldn't question their desire/choice but if they were hesitant or REALLY wanted an opinion I'd be glad to chime in.